The Scribe and the Princess and the Pea

by Jim Turley

OK, perhaps “scribe line” is more accurate, but I do love a double entendre (even if not salacious). I had a discussion with KLA-Tencor at SPIE Litho recently regarding two new machines they’ve just announced. The first allows detection of defects through spectral analysis. The issue it faces is that it relies on test structures in the scribe line, which are facing two challenges: more of them are needed and there’s less space.

More test features are required both because of new structures like the FinFET and new processing steps, double-patterning in particular. But such structures have taken advantage of a generous scribe line area, dictated originally by the width or kerf of actual mechanical saws way back in the day. The cutting is done by laser now, so the kerf is no longer the issue. The scribe line is actually having a measurable impact on dice per wafer, so shrink it must.

The features that their SpectraShape 9000 analyzer looks for are periodic, and their spectra when illuminated by broadband light can be analyzed twelve ways from Sunday. Each of those features goes in a “box” that is currently 45 µm square. To accommodate the smaller scribe line, they’ve reduced the box size to 25 µm on a side (meaning they can almost put four of them where one of the old ones would have gone).

This has come with higher broadband light power, improved sensitivity, and higher throughput for more sampling.

Meanwhile, we’ve come to the point where the smallest (OK, maybe not smallest, but very small) particle – on the backside of the wafer – can push the upper surface out of the depth of field during exposure. Seriously. Total princess-and-pea situation. It gets worse because smaller particles tend to stick harder due to van der Waals forces. And yet such a particle may transfer to the chuck, sharing the donation with the next wafers to come through.

Rather than noticing the effect of such a particle and then going and figuring out where it is, they’ve created a new use model: inspect the backside.* Of each wafer, before it goes into a process. This prevents the particles from ever getting into the chamber – as long as it can be done quickly enough to keep the line moving.

They’ve boosted sensitivity on their BDR300 by 10x to allow for detection of half-micron defects at 100 wafers/hour. They also have a review capability, allowing inspection of defects down to 0.2 µm. It can be integrated into their CIRCL cluster.

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